Title

Author

Degree Type

Dissertation

Date of Award

1991

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

David W. Lynch

Abstract

Photoemission spectroscopy was used to study Ce/Ta(110) and Ce/W(110) interfaces, and the catalytic oxidation effect on both surfaces. A weak Ce/Ta interface reaction is illustrated by the reduced Ta 4f surface core-level shift (SCS) upon Ce adsorption. No Ce/Ta interdiffusion was found. The Ce layer may greatly enhance the oxidation of Ta(110). Oxidation of Ce/Ta(110) at 300 K yields one monolayer (ML) of Ta suboxide (~TaO), followed by the rapid formation of Ta[subscript]2O[subscript]5. Ce/W(110) adsorption patterns were determined by low-energy electron diffraction. For [theta] ≤ 0.5 ML, the adsorption structure is characterized by one-dimensional commensuration along the (110) direction. It changes abruptly to a hexagonal pattern after 0.5 ML. The interatomic spacing shrinks continuously from 9% larger than that for [gamma]-Ce to 3% smaller than that for [alpha]-Ce. Correspondingly the Ce 4f photoelectron spectrum evolves to resemble that of [alpha]-Ce. W 4f core levels are also correlated to the adsorption structure. A significantly widened W SCS is found at low Ce coverages. After the formation of the hexagonal patterns, the Ce-W registration is lost, and the interaction within the Ce layer increases. The SCS for the top layer of W is partially restored. Electronic charge polarization from Ce toward the W (or Ta) substrate is believed to be the key mechanism for the Ce-induced SCS. The interface charge polarization affects deeper layers in the substrate. The W 4f "bulk" component broadens slightly toward lower binding energies, despite no major interface disruption. Oxidation of Ce/W(110) results in the rapid formation of 1 ML of W surface monoxide. No other W oxides were observed. A comparative experiment on O[subscript]2/Ce/W(111) shows the growth of WO[subscript]3 instead of WO. Therefore the surface monoxide formation found on Ce/Ta(110) and Ce/W(110) is not an inherently necessary step in the catalytic oxidation, but rather an interface product on the most densely-packed bcc (110) surfaces. Ce/Ta and Ce/W interface reactions are excluded as the general cause of the catalytic oxidation. An earlier suggestion is reconfirmed that changes in the Ce oxide states convert O[subscript]2 to oxygen ions and thus promote the oxidation of the substrate.